PART III. PHANEROZOIC AEON (570 MYA - PRESENT)

CHAPTER 10.  DEVELOPMENT OF CHORDATES UP TO THE VERTEBRATES

This is the era we later comer, human beings, live in. Aeons are divided into eras. The Phanerozoic aeon is divided into three eras: Paleozoic (570 to 225 mya), Mesozoic (225 to 68 mya), and Cenozoic (68 mya to the present). The era known as the Paleozoic era lasted from 580 to 225 million years ago. But, rather than concentrating on eras we will discuss time in terms of periods, which constitute eras. There are eleven periods in the Phanerozoic aeon.

Chapter ten deals with developments in the Paleozoic era, in which animals ascended to land, shortly afterwards followed by plants. Chapter eleven discusses the age of the reptiles, including the well-known dinosaurs, in the Mesozoic era. And, finally, chapter twelve treats the rise of the primates, including humans, in the Cenozoic era.

PALEOZOIC ERA (570 TO 225 MYA):   THE ASCENT TO LAND

Cambrian Period (570 to 500 MYA)

During the Cambrian Period of the Paleozoic era, the so-called "Cambrian explosion" took place. Animal evolution, especially, made great progress in this period. These were exclusively aquatic animals. All the principle invertebrate types existed: coelenterates (including sea anemones, jelly fish, and hydras), annelids (from which earthworms later developed), molluscs (including mussels, squids, and octopods), and echinoderms (containing the starfish, sea urchins, and sand dollars). More proto-chordates developed. Certain prints found in the Cambrian Burgess strata belong to the proto-chordates.

During this period the tectonic plates were in very different places from today. Close together were South America, Africa and the near east, along with India and Australia, in a supercontinent, known as Gondwana. The eastern edge was at the present-day South Pole, extending northeast from here. North America and Europe were where present South America is today. And Siberia and southern Asia extended southwest from the present

North Pole. From these positions some 570 million years ago, the continental plates drifted towards each other, so that by the Permian period, some 280 million years ago, one supercontinent, known as Pangea, emerged.

Ordovician Period (500 - 440 MYA)

About 500 million years ago, there was enough oxygen in the atmosphere to form an ozone layer that prevented ultraviolet radiation from preventing plants and animals from invading the land. This led to a gradual invasion of the land in the next period.

The Ordovician period saw the development of primitive chordates that eventually led to the development of the vertebrates. These early chordates resembled today's lancelets, because they resembled the point of a lance.

There are four subphyla of vertebrates. During the Ordovician the two acraniate subphyla of the chordates developed. Acraniate means without brain and skull. Only the larvae have notochord and nerve cord. The first phylum is the tunicates. They secrete a tunic, a tough cellulose sac, in which the animal is embedded. The cephalochordates (or lancelets) are very fishlike. They have a notochord and nerve cord that persists in the adult.

Silurian Period (440 - 410 MYA)

In the Silurain period the two remaining subphyla of the chordates developed. These are the vertebrates. The vertebrates are primarily distinguished from the other two groups of chordates by the possession of a vertebral skeleton of articulated (jointed) cartilagenous or bony vertebrae, which replaces the notochord as the axial skeleton. The vertebral skeleton is of great importance because it envelops and protects the central nervous system and provides attachment for muscles; it is stiff and supportive even of a large body, but is still sufficiently flexible to allow sculling motions of the body in swimming and the flexion necessary to terrestrial locomotion. Within the vertebrates there are two major groups, the aquatic, gill-breathing fishes and the terrestrial, lung-breathing tetrapods (four legged animals): the amphibians, reptiles, birds, and mammals.

The subphylum Agnatha contains the agnathids. They lack jaws and paired appendages. They are fish that lack scales and have a round mouth like a suction cup. Modern day examples are the hagfishes and lampreys, which are parasitic on other fish.

The second subphyla that developed was Gnathostomata. These are proto-jawed chordates and real jawless fish. This subphyla contains all the better known vertebrates, including fish, amphibians, reptiles, birds, and mammals.

The Plant Kingdom developed during the Silurian Period. This kingdom derived from the green algae. The first plants are grouped into the general classification of ferns and fern allies. All except the bryophytes (mosses) have vascular tissue (i.e., the system that conducts water and minerals up from the roots to the plant and food from the leaves down towards the roots). About 430 million years ago the phylum Bryophyta (Liverworts, Hornworts, and Mosses) developed. These have a very primitive vascular system. They were the first plants to emerge onto the land.

Devonian Period (410 - 345 MYA)

Around 410 million years ago the first clearly vascular plant invaded the land. This was Cooksonia, which lacked leaves and carried out photosynthesis via its green stems. From Cooksonia developed Zosterophyllumphytina (400 million years ago), which in turn, around 390 million years ago, gave rise to the club and spike mosses. Club Mosses are the first plants to have readily identifiable leaves.

From Cooksonia also developed Rhyniophytina (around 400 million years ago), from which developed Trimeorphytina (375 million years ago). About 370 million years ago phyla Equisetophyta (Horse Tail) and Filicinophyta (Ferns) developed. The Horse Tails have jointed stems. The ferns are more well known.

The vertebrates developed further with the appearance of cartilagenous fish. They have skeletons composed exclusively of cartilage. Then came the development of bony fish that replaced bone for the cartilage. The existence of forty-one orders of bony fishes attests to their environmental success.

Over time the brain stem of fish gradually developed into the full brain. In this process the forebrain gradually enlarged. Accompanying this was the relatively decreasing size of the midbrain. The fish have all the basic parts of the brain that the human has.

The mature vertebrate brain is composed of three parts. The first part, the hindbrain or rhombencephalon, in the fish is concerned with hearing and balance. It contains two parts. The anterior part contains the cerebellum. The cerebellum or "little brain" consists of right and left halves connected to each other, to the cortex of the cerebrum above them, and to the brain stem below by thick bundles of nerve fibers. This organ mainly controls balance and the smooth action of muscles. It also helps parts of the brain above it do their own work in a smooth, efficient manner. The anterior region of the hindbrain also contains the pons. Here the nerve tracts connecting the brain with the spinal cord cross from the left side of the brain to the right side. This crossover explains why the left side of the brain controls the right side of the body, while the right side of the brain controls the left side of the body.

The second part of the hindbrain, the posterior portion, contains the medulla, which is connected to and continues posteriorly as the spinal cord. The medulla coordinates the information received from muscles and joints and the balance organs of the ears. There is a part of the medulla (called the reticular formation) that contains reflex centers for swallowing, vomiting, cardiovascular control, and respiration.

The second part of the brain, the midbrain or mesencephalon, contains the optic lobes, so important to vision for the fish. In fishes and amphibians this is the tectum, an important sensory and motor initiation area, as well as a coordinating center. However, in mammals it is little more than a relay station for visual nerve impulses.

The third part, the forebrain or prosencephalon, was important for smell in the fishes. It contains two parts. The more anterior part contains paired olfactory lobes. These provide the sense of smell. This area also contains the cerebrum, which in humans becomes the massive cerebral hemispheres that cover the rest of the brain and provide reasoning ability. The second part of the forebrain contains the thalamus and hypothalamus. Both organs are important in affecting emotional states. The right and left thalamus share in the control of muscle movements. They also relay and modulate information concerning sensations, such as bodily posture, vision, and pain. The hypothalamus literally means under the thalamus and this actually is its location. This organ serves as a thermostat regulating the internal environment, such as water balance and sleep.

The first land animals were primitive scorpions which later fed on centipedes and millipedes. About 350 million years ago the insects developed. Insects are arthropods, which have an external, jointed exoskeleton, which is very heavy in proportion to the animal's body size. Among the new additions were members of the order Collembola, which contains the springtails. Some 350 million years ago mayflies and cockroaches developed.

Insects have been very successful on Earth in terms of sheer number. Nevertheless, their exoskeleton limited their future evolutionary development. The exoskeleton created for these creatures a problem of exchanging gases with the outer atmosphere. Arthropods developed a tracheal system consisting of a series of air-filled tubes. This system placed an upper limit on the size of the organism.

Other land animals developed from the fishes. One group of fishes developed lunglike devices from outpocketings of the digestive tract, which are known as swim bladders. The fish used the swim bladders for breathing when they became trapped in stagnant pools. In addition, the transition from fins to legs is traced back to a group of fishes called the lobe fins.

Amphibians evolved from the lobe-fins. The Class Amphibia contains frogs, toads, and salamanders. The name amphibian implies that these animals are of the water and of the land, and this is quite true. Very few can live very far from water, and all need water to reproduce. They carry their association with the water via their moist skins. They simply spawn in water and leave their eggs there to hatch.

The brain of the amphibian is not much of an improvement over that of the fish. As in fishes, the midbrain mediates most of the complex behaviors.

It was not until some 250 million years ago that the amphibians differentiated into different lines that eventually became modern-day amphibians and reptiles. Lissamphibia developed into amphibians, Leptospondyli developed into a dead end, and Labrythodontia developed into the reptiles.

Carboniferous Period (345 -280 MYA)

During the Carboniferous Period, huge tree-like forms of Horse Tails, Club Mosses, and Ferns existed. Over time the remains of these huge plants formed the earth's vast coals deposits.

Some 325 million years ago the Progymnosperms developed from Rhyniophytina. Also around 310 million years ago the first true seed plants developed. The advantage of the seed is that it carries its own energy reserves around with it. Therefore, it can last much longer than the spore and prothallus of the fern. It is referred to as a naked seed because it has no covering of fleshy tissue (such as in the latter developing apples and cherries).

The plants with naked seeds are called gymnosperms. There are a number of phyla in this division: Cycads, Ginkgos, and Gnetums. The best know phylum is that of the conifers. This phylum contains the cone-bearing trees, such as the pine tree. The Pine family includes pine, larch, true cedars (Cedrus), fir, spruce, and hemlocks. The Cypress family contains cypresses and junipers. The Taxodiaceae family includes bald cypress, sequoia, dawn redwood, and cryptomeria. The Yew family contains the yews and torreyeas. There are usually separate male and female cones, as in the pines where the female cone is the well-known pine cone and the male cone is usually a smaller, soft cone that releases pollen.

Around 300 million years ago, fossils resembling dragonflies and damselfies were found. Also during the Carboniferous period, amphibians were abundant and reptiles evolved. The reptiles contain the snakes, lizards, turtles, crocodiles, and alligators. These animals made only relatively small advancements over the amphibians. For instance, the reptilian skeleton allows better support for the limbs for rapid movement. The reptiles also developed internal fertilization and an egg, which allowed the young reptiles to develop out of water. The egg conserved water, permitted the embryo to ventilate, and prevented poisoning from its own wastes during development.

In the amphibians and fish the midbrain was important as a mediatory of most complex behaviors. In the reptile the forebrain became more important. Nevertheless, the reptiles kept many of the brain circuits of the fish, such as those that controlled swallowing and elimination. Indeed, in each evolutionary change the higher animals kept the brain circuits of the lower animals that were still useful. In fact, the higher animals even retained those brain circuits that had become entirely useless.

Some new brain developments came with the evolution of the higher reptiles. There appeared a new portion of the cerebrum -- the neopallium (which becomes the major integrating and correlative center in mammals). This organ contains tracts of nerve cell bodies involved with the receipt of sensations other than smell.

Permian Period (280 - 225 MYA)

During the Permian period, one great supercontinent formed when all the continental plates came together. This period of unification proved fleeting as Pangea started to break up soon after it formed. The continental plates then began to drift into their present positions.

Winged insects developed around 300 million years ago. One of these order, the beetles, developed around 280 million years ago. Amphibians still dominated the landscape, but reptiles continued to develop. Around 250 million years ago, mammal-like reptiles exploited new and plentiful insects and small nocturnal vertebrates. A little after the time the dinosaurs appeared, a creature called a proto-avis may have arisen. It may have had a common ancestor with the crocodile. The reptile shoulder evolved into a wing. The alternative theory is that the birds were dinosaurs of the Archaeopteryx type that developed wings and flew. The Archaeopteryx look much like the present day Hoatzin bird of Amazonian South America. This bird has habits possibly similar to those of Archaeopteryx. The babies are the only birds with free toes located in the wings that they use to hang onto tree branches and with claws on the front foot. The young climb trees from which they often dive into the water.

The Permian period ended with a mass extinction, in which the gigantic club mosses and horsetails disappeared. The mass extinction led to the loss of up to 95 percent of all marine invertebrate species. At the same time, deserts became more widespread and the reptiles expanded. The mass extinction made way for the Mesozoic era.

MESOZOIC ERA (225 TO 65 MYA):  AGE OF THE REPTILES

Triassic Period (225 - 190 MYA)

Following the period of mass extinction, the reptiles dominated the landscape. The huge reptiles known collectively as archosaurs, "ruling lizards," and the dinosaurs in particular, came to dominate. Also during the this period, mammals evolved and conifers became abundant. This period also saw the development of those insects known as the true flies (order Diptera), along with the earliest bugs (order Hemiptera). Some 200 million years ago the members of the order Hymenoptera developed. The wasps developed first and from these animals the ants and bees developed. The development of advanced insects would, in turn, lead to the development of flowering plants, which rely on insects rather than wind for pollination.

The discovery of a small paramammal called Thrinaxodon has provided fascinating evidence of the transition in this period to true mammals. But by the end of the Triassic, they and other paramammals vanished. Small hairy mammals, the first true mammals, replaced them. One of the most significant advances of the mammal was in the maternal care of offspring.

Mammals gradually developed three important subclasses. Subclass Prototheria contains the Order Monotremata, of which that odd Australian animal, the platypus, is a member. This creature is an egg-laying mammal with a duck-like bill. Subclass Metatheria contains the marsupials, pouched animals such as the Kangaroo. The higher mammals in the subclass Eutheria developed the placenta, which is a blood-rich lining supplying nutrients to the developing fetus. There are many placental mammals. A few of the orders are Insectivora (shrews and moles); Chiroptera (bats); Carnivora (cats, dogs, bears, and seals); Rodentia (rats, mice, squirrels, porcupines, and beavers); Lagomorpha (hares and rabbits); Artiodactyla (even-toed ungulates, such as cattle and deer); Perissodactyla (odd-toed ungulates, such as horses and rhinoceroses); Proboscidea (elephants); Cetacea (whales and porpoises); and Sirenia (sea cows).

The important thing to realize about the early mammals is how vulnerable they were, since they developed in an age dominated by dinosaurs. The earliest mammals learned to survive by inhabiting all possible environments, even the coldest ones that inhibited reptiles. They survived by hiding in the smallest cracks and crevices, and perhaps by burrowing in the ground. The early mammals survived by being afraid. Their brain made this possible, but would also spell trouble for future mammals, namely humans.

The mammal introduced a new bioprogramming language called emotion. (However, the mammals retained the older, reptilian parts of the brain.) Doctor Paul MacLean (1970) called the new addition, the limbic brain, because, like a limb, it surrounds and embraces the oldest and much smaller, reptilian parts of the brain. The limbic system deals with the emotional feelings that guide behavior. For instance, affection became superimposed over the reptile's sexual drive, and fear became superimposed over the programs that instructed the reptile to stay alive by avoiding larger creatures. (In laboratory experiments, after destruction of parts of the limbic system, young mammals cease to play and the females are deficient in maternal behavior. It is as though these animals regress toward a reptilian condition.)

The limbic brain includes the deep clusters of nerve cells that form the basal ganglia, hippocampus, cingulate gyrus, and the hypothalamus. The basal ganglia are located close to the base of the brain. They vary in size and shape from an almond to a plum, and each has a twin in the opposite side of the brain. Their names are caudate (tail-like), putamen (shell-like), globus pallidus (pale globe), and amygdala (almond). The basal ganglia play an important role in initiating and controlling muscle movements and in modulating and relaying sensations. Some of them also share in the control of emotional reactions.

Parts of the limbic brain itself are responsible for specific emotions. The amygdala influences emotional reactions to odors. Mild electrical stimulation of this nerve-cell cluster causes rats to become suddenly ferocious and violent. Likewise, stimulation of a cluster associated with pleasure causes a cat to purr contentedly.

The hippocampus is from the Greek words hippo (horse) and campus (sea creature). This is because the two structures, one on the right and the other on the left, are the size and shape of a little sea horse. A major function of the hippocampus is its role in memories concerned with emotions such as scents and things past.

Over and around the corpus callosum that unites the two halves of the cerebrum are the cingulate gyrus. Each cingulate gyrus plays a prominent role in regulating the autonomic nervous system, which adjusts blood pressure, heart rate, breathing, the size of the pupils, and other bodily, or psychosomatic, responses that accompany emotional feelings and reactions. The cingulate gyri are also responsible for primitive feelings and expressions of such emotions as despair and the isolation cry of an infant animal deserted by its parents.

Jurassic Period (190 - 136 MYA)

During the Jurassic period, dinosaurs roamed amid vast forests of conifers and ferns. A little after the time the dinosaurs appeared, a creature called a proto-avis appeared. It may have had a common ancestor with the crocodile because the reptile shoulder evolved into a wing. (The alternative theory is that the birds were dinosaurs, known as Archaeopteryx, that developed wings.)

The ability to fly developed from such adaptations as a much lightened skeleton and a powerful respiratory system. One of the most significant advances for birds was the development of warm bloodedness, a characteristic shared with the mammals. This characteristic made these animals more independent of the vagaries of climatic changes.

Cretaceous Period (136 - 65 MYA)

Toward the end of the Cretaceous period, the dinosaurs started to become extinct. One reason for this was that a huge meteor struck the Mexican coast in the vicinity of the Yucatan peninsula. As the dinosaurs declined, the mammals began to change from their nocturnal habits to diurnal ones. Mammals similar to the marsupial opossum came into existence.

Around 112 million years ago, the flowering plants evolved. These angiosperms had what is popularly recognized as a flower (rather than a cone) along with seeds covered with fleshy tissue (i.e., fruits).

CENOZOIC ERA (65 MYA - present) THE RISE OF HUMANS

The earlier part of the Cenozoic, known as the Tertiary, is divided into five distinct epochs, all of relatively brief duration. The last period, known as the Quaternary, takes us up to the present day.

Tertiary Period (65 to 2 MYA)

In the Cenozoic era, all the present-day orders of mammals became established (including some that later became extinct), followed by present-day families and subfamilies. Then appeared the genera, and in some cases, the species that still survive today.

The first part of the Cenozoic era is the Palaeocene epoch (66 to 55 mya). In this epoch mammals rapidly diversified, but were still unlike those alive today. The fossil record shows that the higher insects were undergoing diversification. The rise of the higher orders occurred with the radiation of flowering plants, the two being mutually interdependent. A general trend was that the flowering plants underwent a transition from promiscuous pollination by unspecialized beetles to restricted pollination by specialized insects and other animals.

Arising in the Tertiary period, or perhaps earlier, were the primates. True primates arose some 70 to 65 million years ago. The order Primates contains the humans, as well as lemurs, monkeys, and apes. Especially significant about this order is that it has hands and feet that each have five distinct digits with the innermost toes and thumb usually opposable so that grasping is readily available. The first primates arose in the Paleocence and were very rodent like, resembling the present day tree shrews of southeast Asia. The next ones arose in the Eocene epoch (55 to 37.5 mya) and were comparable to lemuroids and tarsiroids. These later developed into the prosimians.

The next evolutionary step up from the tree shrew-like mammals was the development of the prosimians. These primates contain three families of lemurs, along with indris, the aye-aye, the loris, and bushbabies. Also considered prosimian is the family of tarsiers. Prosimians have small, procumbent incisors. They are usually nocturnal. They also are very reliant on the sense of smell. Unlike the higher primates they do not have color vision, a high degree of stereoscopic vision (promtoing depth perception), and are less social.

The New World monkeys arose some 34 to 12 million years ago, beginning in the Oligocene epoch (37.5 to 24 mya). Except for the Owl Monkey (Aotus trivirgatus), they are diurnal, sleeping mostly in tree holes at night. New world monkeys have a flat nose with two nostrils set widely apart. They usually have a prehensile tail. They live in trees, never descending to the ground. They are widespread through South America and into central America. They are mainly vegetarian, but can feed on insects, worms, and small vertebrates. Examples are the capuchins, marmosets, howlers and the squirrel monkeys.

The old world monkeys developed in the Miocene epoch (24 to 5 mya). They have nostrils set close together and a rather pronounced nose that hangs down. The first one, Colobinae, contains the colubus monkeys, langurs, and proboscis monkeys. The second subfamily, Cercopithecinae, contains the baboons, mandrills, gelada baboons, and macaques.

The superfamily Hominoidea contains the ape families and human beings. (There is some evidence that ape ancestors developed before the old world monkeys.) The apes are in two families, family Hylobatidae, which contains the brachiating gibbons, and family Pongidae which contains the orangutans, gorillas, and chimpanzees.

Occurring in the Pliocene epoch (5 to 2 mya). The final family contains human beings (Homo sapiens). Man will be discussed in following chapters.

Quaternary Period (2 MYA to Present)

This is the period when the genus of the primate Homo becomes strongly established. The evolution of man will be discussed in detail in a later chapter. The Quaternary era is divided into two epochs. The first epoch, the Pleistocene, became about 1.7 million years ago. This is the age when Ice Age mammals were abundant as the ice caps advanced and retreated. The next epoch, the Holocene, which began only .01 million years ago, is the epoch of modern mammals. Human beings increased on all continents.

The Primate Brain

The most recent improvement to the brain in its evolution over millions of years involves its largest, uppermost portion. This is the surface gray matter, or cerebrum (also known as the cortex and neocortex). So that the neocortex would not outgrow the skull, the cerebral cortex wrinkled. The surface literally folded into convolutions so it could fit within the braincase. Some mammals increased in size and enlarged the area of the neopallium disproportionately, so they increased in intelligence as well.

The neocortex became the command post for the deep, "old" parts of the brain. This made it possible for primates to get more, but certainly not complete, control over their more primitive emotions. It also permitted the necessary greater intelligence to solve various situational problems.

 

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